Hi all,
I am building an fm receiver based on the TDA7000 chip which will end up having an output of about 75mV. I will be feeding it into the k-502 amplifier, which has an input impedance of 100KOhms and needs 300mV for full output (need 12dB gain?)...So my question is, should I put in a full preamp, or a small signal amplifier? Also, I will be using the LM1310 to demodulte the signal (should I do this after the gain stage? I think i would prefer it
I'd really like to build something that has a low parts count, and I already have a +/- 15V DC power supply that I could use...
Any suggestions?
Thanks,
Byron
I am building an fm receiver based on the TDA7000 chip which will end up having an output of about 75mV. I will be feeding it into the k-502 amplifier, which has an input impedance of 100KOhms and needs 300mV for full output (need 12dB gain?)...So my question is, should I put in a full preamp, or a small signal amplifier? Also, I will be using the LM1310 to demodulte the signal (should I do this after the gain stage? I think i would prefer it
I'd really like to build something that has a low parts count, and I already have a +/- 15V DC power supply that I could use...
Any suggestions?
Thanks,
Byron
The aim of my question was to see what kind of line driver stage i should build...I mean, a line stage based on the OPA134 should be fairly simple, or maybe something different?
I mean, there are many different ways to go, but i'd like it as simple as possible.
The +/- 15V power supply i have is for a bugle...i wonder, maybe I could use the parts out of the bugle to build a linestage. I think I was using the OPA2134PA.
B
I mean, there are many different ways to go, but i'd like it as simple as possible.
The +/- 15V power supply i have is for a bugle...i wonder, maybe I could use the parts out of the bugle to build a linestage. I think I was using the OPA2134PA.
B
select a discrete output stage using To92 devices.
there are many to choose from.
From +-15Vdc, you can get around 10Vac into a 10k load and possibly 9Vac into a 1k0 load. Running at 20dB gain and averaging 75mVac output leaves the signal well below clipping, quiet when required and undistorted.
there are many to choose from.
From +-15Vdc, you can get around 10Vac into a 10k load and possibly 9Vac into a 1k0 load. Running at 20dB gain and averaging 75mVac output leaves the signal well below clipping, quiet when required and undistorted.
Let's take an extreme case of 4m interconnect having 130pF per metre and a power amp fitted with 1nF RF attenuation caps at the input.
total capacitance seen by the output impedance is 1000pF + 520pF =1520pF.
F-3db=1/2/Pi/R/C/~10kHz, F-1dB~5kHz
That's some treble filter. It should be at least 10times higher and preferably about 20times higher.
i.e. Rs<=500ohms.
total capacitance seen by the output impedance is 1000pF + 520pF =1520pF.
F-3db=1/2/Pi/R/C/~10kHz, F-1dB~5kHz
That's some treble filter. It should be at least 10times higher and preferably about 20times higher.
i.e. Rs<=500ohms.
I have seen commercial CD-players with 600 Ohm output impedance.
An upper limit for line level & small signal audio
is a value of something like 1000 Ohm ( 1 kOhm ).
Such an output impedance would suit most audio amplifiers, pre- or power amplifiers,
with an input impedance of >= 10 kOhm.
Such as 10k, 15k, 22k, 47k or 100k input jacks.
This would satisfy the Rule-Of-The-Thumb (AndrewT mentioned)
that is preferable input impedance is at least 10 times higher than source output impedance.
It wont hurt, of course, if the ratio is x20 or even more ...
But x10 is considered to be acceptable in most cases.
For Line Level Audio.
An upper limit for line level & small signal audio
is a value of something like 1000 Ohm ( 1 kOhm ).
Such an output impedance would suit most audio amplifiers, pre- or power amplifiers,
with an input impedance of >= 10 kOhm.
Such as 10k, 15k, 22k, 47k or 100k input jacks.
This would satisfy the Rule-Of-The-Thumb (AndrewT mentioned)
that is preferable input impedance is at least 10 times higher than source output impedance.
It wont hurt, of course, if the ratio is x20 or even more ...
But x10 is considered to be acceptable in most cases.
For Line Level Audio.
F-3dB =20kHz will sound like a treble filter has been added to everything.CBS240 said:
1/(2*pi*1520pf*20KHz)=5.24K. A bit less than 100K just for this capacitive load.
I always try to over design for the reasonably worst possible scenario.
F-3db >=100kHz is acceptable to most listeners and some prefer as high as 500kHz.
The amp (k-502) has 100k input impedance, and .22uf input capacitors...(schematic is here: http://www.circuitspecialists.com/products/km12sch.jpg)
So would that mean i should try for
1/(2*pi*.22uf*100kHz) = 7.32Ohm output resistance?
That sounds a little low to me...I thought I'd be okay with up to ~5K output impedance...
In any case, I will be going with this (with adjusted R5 for more gain) , since I have all the parts handy:
So would that mean i should try for
1/(2*pi*.22uf*100kHz) = 7.32Ohm output resistance?
That sounds a little low to me...I thought I'd be okay with up to ~5K output impedance...
In any case, I will be going with this (with adjusted R5 for more gain) , since I have all the parts handy:
Attachments
the 0.22uF is the DC blocking high pass filter.
The source impedance interacts with the cable and input capacitance to create a low pass filter.
R5 does not adjust the gain. It isolates the opamp from reactive components in the load.
It also sets the output impedance of the amplifier/buffer.
Change R3 or R4 to alter gain.
Reduce R5 to between 50r and 200r to set Rs in that range.
The source impedance interacts with the cable and input capacitance to create a low pass filter.
R5 does not adjust the gain. It isolates the opamp from reactive components in the load.
It also sets the output impedance of the amplifier/buffer.
Change R3 or R4 to alter gain.
Reduce R5 to between 50r and 200r to set Rs in that range.
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